Direct coupled amplifier



y 1946' Q J. R. BECKWITH 2,400,734

DIRECT COUPLED AMPLIFIER Filed June 12, 1944 INVENTOR.

JOHN R. BECKWITH- /ya z, Qua 17W ATTO R NEYS -ing an anode load impedance.

Patented May 21, 1946 DIRECT COUPLED AMPLIFIER John R. Beckwith, Chicago, Ill., asslgnor, by

mesne assignments, to Belmont Radio Corporation, Chicago, Ill., acorporation of Illinois Application June 12, 1944, Serial No. 539,933

Claims. This invention relates to direct-coupled amplifiersand more particularly to such amplifiers of the type by means of which substantially linear,

distortionless and quiet. amplification may be obtained.

There are many applications in the art for multi-stage direct-coupled amplifiers; that is, amplifiers having a uniform response to all frequencies down to zero frequency or direct current. Such amplifiers are to be distinguished from those employing inductive, capacitive, or

mutual inductive or transformer couplings between the successive stages, all of which coupling elements have non-uniform frequency-response characteristics and the latter two of which do not translate direct-current signals.

Heretofore there have been proposed various direct-coupled amplifiers certain of which have involved operation of the successive stages of the amplifier at progressively higher cathode and anode potentials in order to maintain proper grid biases on the tubes of the several stages. This arangement, however, involves several wellknown disadvantages such as increased shock hazards, the occurrence of spurious signals which are induced in the stages operating substantially above ground potential, and the uneconomic use of the power supply. Other direct-coupled amplifier circuits heretofore proposed have involved the more or less complicated networks. to avoid the disadvantages of the circuit arrangement described, but have resulted in a signal output which was distorted or noisy or both.

It is an object of the invention, therefore, to provide a new and improved direct-coupled amplifier by means of which one or more of the above-mentioned disadvantages of the prior art arrangements may be overcome.

It is another object of the invention to provide a new and improved direct-coupled amplifier in which the successive stages are not operated at progressively higher potentials and in which the overall amplification of each stage is substantially linear, distortionless and quiet.

In accordance with the invention, there is provided in a multi-stage direct-coupled amplifier a substantially linear amplifier stage com prising a vacuum tube amplifier having a signal input circuit and an anode load circuit includ- The stage also includes a vacuum tube repeater having a control electrode circuit and a cathode circuit including a cathode load impedance, the control electrode circuit including a portion of the cathode load impedance. The stage also includes a circuit for impressing anode voltage fluctuations of the amplifier upon the anode of the repeater and a signal output circuit directly coupled to the cathode load impedance and adapted for connection directly to a succeeding amplifier stage.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing while its scope will be pointed out in' the appended claims.

Referring to the single figure of the drawing, there is represented schematically a circuit diagram of a two-stage. direct-coupled amplifier embodying the invention.

Referring now to the drawing, the multi-stage direct-coupled amplifier there represented comprises a substantially linear first amplifier stage It) coupled to a second amplifier stage II. first amplifier stage It comprises a vacuum tube amplifier I2 having a signal-input circuit connected to input terminals I3 across which is connected a grid leak resistor M. The tube I2 is provided with a self-biasing cathode circuit comprising a resistor 15 and a by-pass condenser Hi.

The tube I2 also has an anode load circuit including an anode load impedance, such as a resistor H, connected between its anode and a source of space current, indicated as +3. The amplifier stage l0 also includes a vacuum tube repeater it having a cathode circuit including a cathode load impedance consisting of resistors i9 and 29 in series. The tube i8 is also provided with a control electrode circuit, for example, its control electrode is connected to a example, the repeater l8 may be connected directly in parallel with the amplifier tube I2, as indicated. The stage also includes a signaloutput circuit including terminals 2| directly coupled across the cathode load impedance consisting of resistors l9 and 29; this signal-output circuit. as explained hereinafter, being adapted for connection directly to the succeeding amplifier stage II. The amplifier tube l2 may be of any suitable type, there being illustrated by way of example a screen-grid tube the screen electrode of which is connected to the terminal +B through a dropping resistor 22 and is by-passed for signal-frequency currents by a condenser 23.

The Y The second amplifier stage H is shown as comprising a conventional screen grid amplifier tube 24 connected to the source +B through an anode load resistor 25 and provided with a cathode bias resistor 26 by-passed by condenser 21. The bias on the cathode of the tube 24 is determined by means of a voltage divider comprising a resistor 28 and the resistor 2 6 connected in series across the source +B. The tube 24 is il1ustratcd as also being of the screen grid type, its screen electrode being connected to the source +B through a dropping resistor 29 and being by-passed to ground for signal-frequency currents by means of a condenser 30. The output circuit of the amplifier stage Ii includes the terminals 3] connected across the anode load resistor 25 of vacuum tube 2|, as indicated.

In the operation of the direct-coupled amphfier system described, the vacuum tube l2 and its associated circuit elements operates in a conventional manner to amplify the signal applied to the input terminals l3. The amplified signal output appears at the anode of the tube 12 and this signal is applied directly to the anode of the vacuum tube repeater l8. The connection of the control electrode of the tube 18 to the junction of the resistors l9 and 20 serves two pur-- poses: first, the resistor l9 provides an appropriate bias to the control electrode of tube [8, and second, since the resistor 19 is not by-passed,

the connection of the control electrode of tube- IB is effectively a degenerative one which imparts to the tube ill a non-linear resistance characteristic. Specifically, increases in the anode potential of the tube l8 effectively increase the space current through the tube and the cathode load resistors 19 and 20, thus increasing the negative instantaneous bias onthe grid of the tube l8 and increasing its efiective resistance. As a result, the current fluctuationsthrough this circuit are partially degeneratedand the signal that the final amplifier stage be a conventional one, as represented by the stage I I, since the portionof the stage IB including the repeater l8 actually attenuates the signal so that the gain of stage 10 is less than that of a stage of the type 4 of stage II.

It will be apparentthat the circuit constants I of the direct-coupled amplifier system represented in the drawing may beselected in accordance, with the design requirements of any particular installation. By way of example, however,

the following are givenas representative of a particular amplifier stage:

Tube 12 ...r Type 6V6 Tube l8--- Type 6SN'7 (one space-current path) Tube 24 Type 6116 Resistor l9 820 ohms Resistor 20 10,000 ohms Resistorl1 18,000 ohms In a direct-coupled amplifier embodying the circuit constants given above, the repeater tube I8 was determined to provide an amplification factor of 0.35;

While there has been described what is at present considered to be the preferred embodi skilled in the art that various changes and modivoltage appearing across the cathode load circuit A of resistors 19 and 20, this compensation of nonlinearity may bemade very nearly exact.

The signal output of the stage l0 appearing at the terminals 2! is a uni-directional signal having an average positive value, but still one of low enough value that it may be applied directly to the control'electrode of the amplifier tube 24 or stage II. Nevertheless, in order to insure a proper operating bias on the control electrodeof this tube, its cathode is biasedpositively by means of the voltage divider comprising resistors 28 and 26 in series across the source +B. The amplifier stage II operates in a conventional manner to supply to its outputterminals 3| an amplified undistorted reproduction of the signal input applied tothe terminals l3 of stage III.

While the invention has been illustrated as embodied in a two-stage direct-coupled amplifier, it will be understood that any desired number of amplifier stages similar to the stage In may be connected in cascade. However, it is preferable ment of the invention, it will be apparent to those fications may be made therein without departing from the spirit or scope of the invention. What is claimed as new is:

1. In a multi-stage direct-coupled amplifier, a

substantially linear amplifier stage comprising, a vacuum tube amplifier having a signal-input circuit and an anode load circuit including an anode load impedance, a vacuum tube repeater having a control electrode circuit and a cathode circuit including acathode load impedance, said' control electrode circuit including a portion of said cathode load impedance, a circuit for impressing anode-voltage fluctuations of said amplifier upon the anode of said repeater, and a signal -output circuit directly coupled to said cathode load impedance and adapted for connection directly to a succeeding amplifier stage.

2. In a multi-stage direct-coupled amplifier, a substantially linear amplifier stage comprising, a vacuum tube amplifier having a signal-input circuit and an anode load circuit including an anode load impedance, a vacuum tube repeater connected in parallel with said vacuum tube amplifier and having a control electrode circuit and a cathode circuit including a cathode load impedance, said control electrode circuit including a portion of said cathode load impedance, and signal-output circuit directly coupled to said cathode load impedance and adapted for onnection directly to a succeeding amplifier stage.

3. In a multi-stage direct-coupled amplifier, a

substantially linear amplifier stage comprising, a vacuum tube amplifier having a signal-input circuit and an anode load circuit including an anode load impedance, a vacuum tube repeater having a control electrode and a cathode circuit including a cathode load impedance, a connection from a point on said cathode load impedance to said control electrode, a circuit for impressing anode-voltage fluctuations of said amplifier upon the anode of saidrepeater, and a signal-output circuit directly coupled to saidcathode load impedance and adapted for connection directly to a succeeding amplifier stage.

4. A multi-stage direct-coupled amplifier comprising, a substantially linear first amplifier stage including a vacuum tube amplifier having circuit and an anode load circuit including an anode load resistor, a vacuum tube repeater having a control electrode circuit and a cathode circuit including a cathode load resistor, said control electrode circuit including a portion of said cathode load resistor, a circuit for impressing anode-voltage fluctuations of said amplifier upon the anode of said repeater, and a. signal-output circuit directly coupled to said cathode load resistor and adapted for connection directly to a succeeding amplifier stage.

- JOHN R. BECKWI'I'H. 

